In general, when a telephone with a spot dial function is used while a third party is calling the telephone, the telephone company will send a multi-frequency signal to tell the user the spot dial coming. This is the so-called CAS (CPE (Customer Premise Equipment) Alerting Signal) tone. When the telephone circuit detects this signal, an acknowledged signal will be sent to the telephone company. The telephone company may send the telephone number of the third party and relative messages to the user telephone. The CAS tone comprises the multi-frequency signal of 2130 HZ and 2750 HZ. When the caller-ID circuit receives the CAS tone, both the signal frequency and the signal length will be detected. If the frequency and length of the signal meet the specification, the signal may be acknowledged as the signal sent from the telephone company. The Bellcore specification is listed in table 1.
The conventional detection circuit has a damping effect when a CAS tone having large power is received. The effect causes the CAS tone received to be longer than the real CAS tone transmitted, so the CAS tone detection may be incorrect.
For the example of the CAS tone with high frequency 2750 HZ, when the 2750 HZ signal of the input signal is filtered through a band pass filter, the level of “1” or “0” will be decided by a comparator, and such a procedure is the so-called analog to digital conversion. The comparator output is dependent on the hysteresis. The principle is similar to the schmitt trigger. FIG. 1 shows the characteristic curve, wherein Vref is the reference voltage in the analog circuit and the voltage potential from Vth− to Vth+ is the hysteresis value. When the Vi is increased, if Vi>Vth+, Vo=“1”, and when the Vi is decreased, if Vi<Vth−, Vo=“0”. The hysteresis value is based on the minimum value acceptable in the telecommunication specification. The signal will be sent to the detection circuit through the hysteresis comparator. The detection circuit consists of counters used to detect the valid input signal according to the signal frequency. If the input signal is valid, the detection circuit outputs CAS_ALGO=“1”, if not, the CAS_ALGO=“0”.
FIG. 2 shows the conventional circuit. A short damping effect will appear in case of a large input signal through the band pass filter 21 or 22 after the input signal is ended, but this wrong input signal through the detection of hysteresis comparator 23 or 24 may appear as a valid signal. The larger the signal strength is, the longer the time taken. In speaking of the filter, the calculation time of the digital filter is longer than the analog filter, so the damping effect will be more serious with the digital filter.